EXPR(1) BSD General Commands Manual EXPR(1)NAME
expr -- evaluate expression
SYNOPSIS
expr expression
DESCRIPTION
The expr utility evaluates expression and writes the result on standard output.
All operators are separate arguments to the expr utility. Characters special to the command
interpreter must be escaped.
Operators are listed below in order of increasing precedence. Operators with equal prece-
dence are grouped within { } symbols.
expr1 | expr2
Returns the evaluation of expr1 if it is neither an empty string nor zero; other-
wise, returns the evaluation of expr2.
expr1 & expr2
Returns the evaluation of expr1 if neither expression evaluates to an empty string
or zero; otherwise, returns zero.
expr1 {=, >, >=, <, <=, !=} expr2
Returns the results of integer comparison if both arguments are integers; otherwise,
returns the results of string comparison using the locale-specific collation
sequence. The result of each comparison is 1 if the specified relation is true, or
0 if the relation is false.
expr1 {+, -} expr2
Returns the results of addition or subtraction of integer-valued arguments.
expr1 {*, /, %} expr2
Returns the results of multiplication, integer division, or remainder of integer-
valued arguments.
expr1 : expr2
The ``:'' operator matches expr1 against expr2, which must be a regular expression.
The regular expression is anchored to the beginning of the string with an implicit
``^''.
If the match succeeds and the pattern contains at least one regular expression sub-
expression ``\(...\)'', the string corresponding to ``\1'' is returned; otherwise
the matching operator returns the number of characters matched. If the match fails
and the pattern contains a regular expression subexpression the null string is
returned; otherwise 0.
( expr )
Parentheses are used for grouping in the usual manner.
Additionally, the following keywords are recognized:
length expr
Returns the length of the specified string in bytes.
Operator precedence (from highest to lowest):
1. parentheses
2. length
3. ``:''
4. ``*'', ``/'', and ``%''
5. ``+'' and ``-''
6. compare operators
7. ``&''
8. ``|''
EXIT STATUS
The expr utility exits with one of the following values:
0 the expression is neither an empty string nor 0.
1 the expression is an empty string or 0.
2 the expression is invalid.
>2 an error occurred (such as memory allocation failure).
EXAMPLES
1. The following example adds one to the variable a.
a=`expr $a + 1`
2. The following example returns zero, due to subtraction having higher precedence than
'&' operator.
expr 1 '&' 1 - 1
3. The following example returns the filename portion of a pathname stored in variable a.
expr /$a : '.*/\(.*\)'
4. The following example returns the number of characters in variable a.
expr $a : '.*'
COMPATIBILITY
This implementation of expr internally uses 64 bit representation of integers and checks for
over- and underflows. It also treats / (division mark) and option '--' correctly depending
upon context.
expr on other systems (including NetBSD up to and including NetBSD 1.5) might not be so
graceful. Arithmetic results might be arbitrarily limited on such systems, most commonly to
32 bit quantities. This means such expr can only process values between -2147483648 and
+2147483647.
On other systems, expr might also not work correctly for regular expressions where either
side contains single forward slash, like this:
expr / : '.*/\(.*\)'
If this is the case, you might use // (double forward slash) to avoid confusion with the
division operator:
expr "//$a" : '.*/\(.*\)'
According to IEEE Std 1003.2 (``POSIX.2''), expr has to recognize special option '--', treat
it as an end of command line options and ignore it. Some expr implementations don't recog-
nize it at all, others might ignore it even in cases where doing so results in syntax error.
There should be same result for both following examples, but it might not always be:
1. expr -- : .
2. expr -- -- : .
Although NetBSD expr handles both cases correctly, you should not depend on this behavior
for portability reasons and avoid passing bare '--' as first argument.
STANDARDS
The expr utility conforms to IEEE Std 1003.2 (``POSIX.2''). The length keyword is an exten-
sion for compatibility with GNU expr.
AUTHORS
Original implementation was written by J.T. Conklin <jtc@NetBSD.org>. It was rewritten for
NetBSD 1.6 by
Jaromir Dolecek <jdolecek@NetBSD.org>.
NOTES
The empty string ``'' cannot be matched with the intuitive:
expr '' : '$'
The reason is that the returned number of matched characters (zero) is indistinguishable
from a failed match, so this returns failure. To match the empty string, use something
like:
expr x'' : 'x$'
BSD April 20, 2004 BSD